[FEATURE] Make use of the Visible page information

If the meta information Visible is set to False for a page,
then will handle the as if it dosn't exists and a 404 status will be
thrown.
[FEATURE] Render the content of a specific path

Adds the path argument to the AbstractMdViewhelper. This will
make it possible to render the content of a path and not just the current page.
Merge pull request #5 from Kollode/master

Some follow-up changes